December 11, 2006
01:30 PM (EST)

News Release Number: STScI-2006-54

Heavyweight Stars Light Up Nebula NGC 6357

December 11, 2006: The small open star cluster Pismis 24 lies in the core of the large
emission nebula NGC 6357 in Scorpius, about 8,000 light-years away
from Earth. Some of the stars in this cluster are extremely massive and
emit intense ultraviolet radiation. The brightest object in the picture is
designated Pismis 24-1. It was once thought to weigh as much as 200 to
300 solar masses. This would not only have made it by far the most
massive known star in the galaxy, but would have put it considerably
above the currently believed upper mass limit of about 150 solar masses
for individual stars. However, high-resolution Hubble Space Telescope
images of the star show that it is really two stars orbiting one another (inset
pictures at top right and bottom right). They are estimated to each be 100
solar masses. The Hubble Advanced Camera for Surveys images were taken
in April 2006.